Influence Of Iron-based Flocculants On Acid Production Performance Of Alkali-treated Sludge By Fermentation

At present,insufficient carbon source in the influent of the sewage treatment plant is the main reason that affects the performance of the biological nitrogen and phosphorus removal process and the stability of the effluent.The residual sludge of the sewage treatment plant is rich in a large amount of organic matter.If sufficient amount of volatile fatty acids(VFAs)can be recovered from the sludge by hydrolysis and fermentation as the internal carbon source,it can effectively solve the problem of insufficient carbon source and sludge disposal.However,since the organic matter in the residual activated sludge is basically composed of microbial cells,the acid production efficiency of fermentation is low,and alkali pretreatment is often used to destroy the cell structure of the sludge and accelerate the process of hydrolysis and acidification.In addition,in order to achieve a higher phosphorus removal effect,iron-based flocculants are often used in sewage plants to form iron-containing sludge.Whether the use of iron-based flocculants will affect the acid production efficiency of alkaline treatment sludge fermentation,and how to improve the acid production effect of iron-containing sludge through process optimization deserves further study.Therefore,in this paper,on the basis of investigating the effect of alkali pretreatment to improve the acid production of sludge,the effects of different dosages of two commonly used iron-based flocculants(FeCl3·6H2O,Polyferric sulfate PFS)on the effect of alkaline pretreatment sludge fermentation on acid production were discussed.The acid production efficiency of iron-rich sludge was optimized by semi-continuous experiments,and the feasibility of using fermentation broth for denitrification of sewage was experimentally analyzed.Finally came to the following conclusions:(1)Although the use of iron-based flocculants increased the SCOD output of the alkali-treated sludge,it reduced the acid production efficiency.Compared with the untreated sludge,the maximum acid production efficiency of the sludge was increased by 12.35 times after alkali treatment.The iron-containing sludge(FeCl3 sludge)formed by using ferric chloride after alkali treatment(equivalent to FeC13 6H2O dosage of 25 mg Fe/g VS),compared with the sludge without ferric chloride(alkali treatment),the maximum production of SCOD increased by 21.3%,but the maximum yield of VFA decreased by 45.64%.This is mainly due to the rapid consumption of VFA due to the use of ferric chloride to promote methanogenesis.However,appropriately reducing the usage of ferric chloride can alleviate the VFA consumption process to a certain extent.When the dosage of FeC3·6H2O is 20mg Fe/g VS,the maximum VFA yield of the sludge treated with the alkali treatment is the same as that of the iron-containing sludge treated with the alkali treatment(equivalent to the dosage of FeCl3·6H2O being 25mg Fe/g VS)Compared with the increase of 21.6%.(2)The effect of ferric chloride on acid production efficiency is less than that of Poly ferric sulfate.The use of PFS will also cause the reduction of acid production efficiency of alkali treated sludge,but the effect of PFS sludge on the reduction of acid production efficiency is slightly greater than that of FeCl3 sludge.The use of PAM in the water treatment system has basically no significant effect on the acid production efficiency of alkaline treatment of iron-containing sludge.Therefore,it is feasible to use alkaline treatment of FeCl3 sludge for fermentation and acid production as the internal carbon source of the sewage treatment plant,but the dosage of ferric chloride in the phosphorus removal process of the sewage treatment plant should preferably be controlled within 20mg Fe/g VS.(3)Through semi-continuous experiments,the process optimization parameters for acid production by alkaline treatment of iron-containing sludge were obtained.After the iron-containing sludge formed by the appropriate use of ferric chloride(the dosage is 20mg Fe/g VS),after alkali treatment and anaerobic fermentation,when the dosing ratio is 12.5%,the solid concentration is 2.5%,and the solid residence time is 2 d,the yields of SCOD and VFA were maintained at 2200-2600 mg/L and 270-370 mg/L,respectively.the output of VFA can basically meet the carbon source requirements for nitrogen and phosphorus removal in the sewage treatment plant,indicating that the VFA produced by the fermentation of iron-containing sludge in the alkaline treatment can be used as a stable internal carbon source in the sewage treatment plant.However,after the anaerobic fermentation of iron-containing sludge formed by using PFS(the dosage is 15 mg Fe/g VS),the acid production effect of the system is unstable,which will affect the engineering application of subsequent anaerobic fermentation.(4)It is feasible to use sludge fermentation broth as a carbon source for denitrification and denitrification.The fermentation broth produced by the fermentation of ferric chloride sludge with alkali treatment was used as the carbon source,and the removal rate of TN was 8%,while the removal rate of TN was 15%when sodium acetate was used as the carbon source.However,the removal of TN from the fermentation broth(14.418 mg/L)was 2.7 times that of sodium acetate as the carbon source.Therefore,it is feasible to use sludge fermentation broth as carbon source for denitrification.